The present invention relates to a microcomputer.
Recently, in accordance with a strong demand for higher performance microcomputers, an increase in the effective number of instructions executable by a microcomputer has been required.
In the case of an 8-bit microcomputer, the number of the executable instructions is 256, since each instruction is composed of 8 bits, whose contents distinguish one instruction from another, and 2.sup.8 =256. Usually, a microcomputer has addressing modes to realize an ease-of-use in flexibly accessing different addresses. Since a certain number of bits out of total 8 bits are assigned and used for designating the addressing modes, the effective number of instructions, or the number of instruction categories, or the number of kinds of instructions executable by the 8-bit microcomputer becomes far less than 256. On the other hand, as mentioned above, as the demand for higher performance microcomputers increases a migration from 8-bit microcomputers to 16-bit, 32-bit and even 64-bit microcomputers is happening in the industry. From the standpoint of the microcomputer users, this migration should be accompanied by a certain compatibility and uniformity to realize a systematic migration.
In other words, a series of microcomputer models from 8-bit to 64-bit is required in order to base a systematic concept common throughout the series.
The above requirement can be achieved by increasing the kinds of instructions, without modifying the processing unit of the CPU; in other words, by facilitating incorporation of .mu.OS (operating system) with the basic 8-bit microcomputer. For this purpose, a method of increasing the number of bits which constitute one word can be considered. That is, in the case of an 8-bit microcomputer, when the second byte is used to designate the kinds of instructions in addition to the first byte, it is possible to increase the number of instructions that can be executed from 2.sup.8 =256 to 2.sup.16 =65536.
In this method, although the number of instructions that can be executed can be definitely increased, there exists a problem in that the processing speed is reduced by half and therefore the execution time is doubled, because the CPU must read an instruction twice, 8 bits by 8 bits, in each instruction execution. In addition, there exists another problem in that the capacity of the ROM must be increased both with the increasing number of instructions and the increasing number of bits constituting one word.
As described above, although it has been so far considered that the kinds of instructions can be increased by simply increasing the number of bits constituting one word, this inevitably results in an increase in execution time and ROM usage.